When, in general, a monochrome photograph is to be enlarged by a small-sized photographic enlarger, a projection lens of a focal length is selected by the operator in compliance with the desired enlarging magnification, and attached to an enlarger head in or through which a negative or positive film referred to as original film as including both negative and positive films is set. Then, upon appropriate determination of the distance between the head including a light source, the film and the projection lens and a surface of a printing paper, the focus of the projection lens and/or the diaphragm aperture are adjusted to desired values, thereby a desired state of an enlarging optical system being established. Thereafter a trial printing is made several times to determine the amount of light to be emitted from the light source of the photographic enlarger for obtaining a desired print. In a photographic enlarger of a conventional type having its lamp energized continuously during exposure to emit constant intensity light, the amount of light from the lamp can be determined by the period of the lamp energization, i.e., the exposure time. Then the operator may memorize the exposure time for the best print and manually control the period of the lamp energization in accordance with his or her memory. Otherwise, he or she may set the exposure time on a timer which measures the time and automatically controls the exposure time to a set value. When, however, the distance between the light source and the printing paper surface and/or the diaphragm aperture of the projection lens are changed or the frame of the original film is replaced by another frame having a different average density, the condition of the enlarging optical system is renewed, and trial printings should be made again to find out a proper exposure time for the renewed state of the enlarging optical system.
In enlarging a color photograph, color balance or distribution of the light emitted from the light source and entering the original film should be taken into consideration. In the case of printing with a conventional small-sized color photographic enlarger, the trial printings are made by the operator changing the printing conditions until the condition for obtaining a best print is found for a picture frame of an original film (hereinafter referred to as "the best print condition"). Then, with the enlarger set to the best print condition, the light transmitted through the original film is measured and memorized in, for example, a so-called negative color analyzer for determining data of the exposure amount with respect to each of three primary colors. Then the original film is replaced by another film and the printing conditions such as the quantity of light emission from the light source and the color balance are relatively adjusted in accordance with the indication of the original color analyzer which memorizes the best print conditions. After the adjustment, actual printing operation is carried out. In such a device, however, the operational process from the trial printing to the actual printing operation is so complicated that the device might be erroneously operated. Further, in a case where the condition of the enlarging optical system is changed from the best print condition, e.g., when the size of the diaphragm aperture of the projection optical system is changed or the enlarging magnification is changed, such a complicated process must again be repeated from the trial printing for finding out the best print condition for a standard original film or otherwise the printing conditions are calculated by the operator as a function of the changes in the enlarging optical system.
The color analyzer for the photographic enlarger of the aforementioned type is used in the manner that after the best print conditions established in the trial printing operation are memorized, the condition of the light source in actual printing operation is manually adjusted by the operator who confirms whether the adjusted conditions are conformed to the memorized conditions or not by observing a display of the analyzer. Thus, operation of the analyzer in a darkroom is also complicated and inconvenient and such an analyzer can not provide any suitable solution to the problem of complexity of the printing operation when the condition of the enlarging optical system is changed.
In a processing laboratory using a large-sized photographic enlarger for mass production, a printing condition with which best prints are generally obtained is previously set in the enlarger and then the light transmitted through the film is measured by an original color analyzer which calculates the condition of the light source for its light emission. The data thus calculated are stored in a magnetic recording tape so that exposure is made on printing papers with the light source being controlled on the basis of the data provided from the magnetic recording tape. However, said generally predetermined best print condition is set on the premise that the enlarging optical system such as the enlarging magnification is not changed, and the device cannot cope with changes in the enlarging optical system. If the device is to be so constructed as to meet said changes, the construction thereof will be more complicated.
The aforementioned conventional photographic enlargers are further disadvantageous in that intensity of light to be emitted from the light source must be set constant similarly at the time of the trial printing operation and at the time of actual printing, requiring careful maintenance of the light source.